The Agriculture Department (search) said Monday it is ending its search for additional cases of mad cow disease (search) even though officials have not found all the animals they sought after the nation's first case turned up in December.
"Our investigation is now complete," Ron De Haven, the department's chief veterinarian, told reporters in a telephone conference call. "We feel very confident the remaining animals, the ones we have not been able to positively identify, represent little risk."
The closure leaves officials not knowing what happened to 11 head of cattle among 25 that authorities consider likely to have eaten the same potentially infectious feed given to Washington state Holstein that tested positive for mad cow in December.
All 25 were among 81 born on a farm in Alberta, Canada, and shipped into the United States in 2001. Officials have found 29 of the 81, including 14 considered most at risk.
The search for the 81 cattle led authorities to 189 farms and ranches and the testing of 255 animals, none of which had bovine spongiform encephalopathy or BSE, the technical name for mad cow, De Haven said. Some may have gone to slaughter, but BSE tests would have spotted any that had mad cow, he said.
The likelihood of finding more cases "is pretty slim at this point," De Haven said.